Abstract Delirium and sleep disruption are both common in the older patients who are hospitalized. Delirium is a state of acute confusion, experienced especially by older adults admitted to the hospital, with the potential to adversely impact patients' outcome. Of hospitalized patients, one of the highest rates of delirium occurs after major surgery. Development of postoperative delirium is associated with longer hospital length of stay, significantly higher risk of functional decline, loss of independent living, and increased mortality. Previous studies have focused on describing the clinical manifestations and outcomes of postoperative delirium; yet, whether sleep disruption is associated with the development of delirium, has not been rigorously investigated. Sleep disruption, including changes in sleep patterns and architecture, and decreased quality of sleep are commonly observed in older subjects. In the hospital, environmental factors and health care practices further contribute to sleep disruption in the postoperative older patients. In preliminary data acquired from older surgical patients who were monitored with continuous wrist actigraphy, we observed that sleep disruption as measured by total wake time was significantly longer, and number of awakenings was significantly higher even before surgery. The pattern of sleep disruption observed before the planned surgery was associated with increased rates of postoperative delirium. In contrast to previous small studies investigating sleep and cognitive outcomes, we propose a comprehensive, exploratory prospective cohort study to test the hypothesis that preoperative sleep disruption is an independent predictor for postoperative delirium after adjusting for known risks for postoperative delirium. Also, in contrast to previous studies which focused only on the period after surgery, we propose to test the hypothesis that sleep disruption in older patients is prevalent before surgery and is associated with modifiable patient-related risk factors. In patients aged 65 years of age or older undergoing noncardiac surgery, we will measure sleep before and after surgery continuously using wrist actigraphy. Postoperative delirium will be measured daily with a valid and reliable measure. Our overall goal is to determine the relationship between sleep disruption before and after surgery in order to determine the optimal time period for intervention. In addition, we will also measure the risk factors associated with sleep disruption at home and after surgery to enable the development of strategies to improve sleep and minimize delirium. Our study results will be critical for designing future randomized clinical trials to target both patient- related and hospital-related factors that precipitate sleep disruption, with the ultimate goal to improve sleep hygiene and decreasing adverse postoperative cognitive outcomes.